The present invention relates to the domain of the production of low-nitrogen steels. It is advantageously applied to the production of low and very low-carbon grades.
It is known that the presence of nitrogen in steel can prove undesirable for different reasons. One of them is the impact of this element on the properties of use of the steels, further to a reduction in the ductility of the metal and therefore in its aptitude to stamping, or, if the nitrogen is present in the form of aluminium nitrides, further to a limitation of the weldability due to a redissolution of the nitrogen in the ZAC (heat-affected zone) and the resultant local mechanical fragilization. However, the presence of nitrogen can also be undesirable by reason of its impact on the very steps of the production procedures, such as an increase in the cracks linked with the ductility pocket at continuous casting, or the reduction in the aptitude of the product obtained to be wire-drawn.
The processes of production, or the grade of certain steels, therefore sometimes require very low nitrogen contents on the final product obtained, for example, to give an idea, from 15 to 25 ppm for sheets intended for automobile construction or for steels for packing, of about 50 ppm for offshore platform plates, or from 40 to 60 ppm for tyre-reinforcing wires, etc. . . . These nitrogen contents are expected in the steelworks, at all stages of production of the molten metal, from the electric oven, or from the converter, up to its solidification at continuous casting. It is known that the production in the electric oven, in particular, is distinguished by a considerable contamination of the metal with nitrogen, due to the cracking of the molecule of nitrogen of the air in the heat zone of the electric arc which facilitates its transfer to the liquid metal. This phenomenon is known to be an important factor which prevents production by the xe2x80x9celectrical procedurexe2x80x9d of a part of the grades produced today by the xe2x80x9ccasting procedurexe2x80x9d (reduction-melting of the molten iron ores in the blast furnace then refining with oxygen in a pneumatic converter) by which lower nitrogen contents, of the order of 20 ppm, are currently obtained.
The physico-chemical mechanisms which govern the evolution of the nitrogen content in liquid steel are well known (cf. for example the article by Ch. Gatellier and H. Gaye in the REVUE de METALLURGIE, CIT of January 1986, pp 25-42). The nitrogen follows a xe2x80x9cmetal-gasxe2x80x9d chemical equilibrium which may be expressed by the formula N⇄xc2xd N2(gas). The constant of equilibrium of this reaction, which is written KN=aN/(PN2)xc2xd, depends slightly on the temperature in the operational domain of the reactors concerned (1550 to 1700xc2x0 C.). aN is the activity in dissolved nitrogen, which may be assimilated to the nitrogen content of the metal in the case of the weakly alloyed carbon steels, and PN2 is the partial nitrogen pressure of the gas in contact with the liquid metal. This means that, in the presence of atmospheric N2, the nitrogen content of the metal will continually increase towards its limit of solubility, which lies in the neighbourhood of 430 ppm at the temperature of the molten steel (about 1600xc2x0 C.).
As for the denitriding of the metal, it is obtained by circulating in the liquid metal a washing gas not comprising nitrogen (PN2=O) in order to displace the afore-mentioned reaction towards the right (washing effect). Industrially, this gas may be injected argon or helium, but at low flowrate and with a high cost, or carbon monoxide formed in situ by the decarburization of the metal during the injection of oxygen, which is conventionally practised in gaseous or particulate form (cf. for example the article by K. Shinme and T. Matsuo: xe2x80x9cAcceleration of nitrogen removal with decarburization by powdered oxidizer blowing under reduced pressurexe2x80x9d, in the Japanese Journal ISIJ in 1987). The limit to this practice of injection of O2 is linked with the carbon content of the metal at the beginning of decarburization, which will impose the volume of CO emitted in the course of time and therefore the possible denitriding, and this whatever the initial and envisaged nitrogen contents of the metal to be produced.
This physico-chemical approach must be completed by the role performed by the surface-active elements of the metal, namely oxygen and sulphur, which both have the effect of blocking the transfers of nitrogen between metal and gas. Consequently, beyond a certain activity in dissolved oxygen, corresponding to an upper limit of the carbon content which is of the order of 0.1% by weight for carbon steels), the denitriding by washing gas may be totally inhibited.
All the interest in being able to develop a technique of denitriding of the liquid metal, making it possible in particular to produce by the xe2x80x9celectricalxe2x80x9d procedure steels whose nitrogen contents are similar to those obtained by the xe2x80x9ccastingxe2x80x9d procedure, i.e. of the order of 20 ppm, and even less on the final product obtained, is thus understood.
The purpose of the present invention is precisely to promote a denitriding of the molten metal which best exploits the denitriding potential of the washing gas, on the one hand, and which, on the other hand, makes it possible to control the final nitrogen content independently of the initial carbon content of the metal bath, while this is presently the case with a conventional decarburization.
To that end, the invention has for its object a method for denitriding molten steel during its production by blowing oxygen, characterized in that it consists in likewise introducing carbon in a form capable of being blown (powder carbon), and in that carbon and oxygen are injected jointly but separately into the same metal bath zone (at some 20 cm distance from each other, for example).
In the carbon and oxygen input zone, conditions favourable to denitriding are thus locally created. In effect, in the case of a simple injection of oxygen (case of conventional decarburization), the injection zone (nozzle) will be translated rapidly by an impoverishment of carbon which will delay the formation of CO, and by a correlatively high activity in dissolved oxygen which, as is known, will act against the denitriding of the metal by the CO bubbles formed.
The combined input of carbon in this same zone will allow a more rapid formation of the CO bubbles by reaction between carbon and oxygen introduced, and a reduction of the local activity in dissolved oxygen. Consequently, there is obtained a better efficiency of the denitriding by the CO emitted , which will thus supplant the natural tendency of the steel to be nitrided upon contact with the nitrogen of the surface air and therefore lead overall to a reduction of the nitrogen content of the metal.
In effect, it is recalled that, in an arc furnace, like, moreover, in any metallurgical reactor which composes the procedure for producing the metal, the enclosure is not and cannot be strictly tight with respect to the outside atmosphere. Consequently, the final nitrogen content of the product obtained necessarily results from a compromise between the nitrogen regains (contamination by the air, for example) and the denitriding carried out during production in the liquid state.
Furthermore, by preferably adjusting the inputs stoichiometrically (namely 1 kg of C for 0.9 Nm3 of O2), the carbon content of the metal bath is not modified. In this way, an emission of CO with xe2x80x9cconstant carbon content of the bathxe2x80x9d is thus effected, and whose duration may then be adapted to the desired denitriding (nitrogen content envisaged with respect to the initial nitrogen content).